1. Field of the Invention
The present invention is related to a method of transmitting data in a packet based network, and/or to a device for controlling a transmission of data in a packet based network, and/or to a communication network system using a corresponding device, and or to a corresponding computer program product. The following specification is in particular directed to a queuing/scheduling mechanism used in a transmission of packets of data for optimizing transmission performance in communication networks.
2. Description of the Related Prior Art
In the last years, an increasingly extension of data transmission networks, such as the Internet, and communication networks, such as wire based communication networks like the Integrated Services Digital Network (ISDN), or wireless communication networks like cdma2000 (code division multiple access) system, Universal. Mobile Telecommunications System (UMTS), the General Packet Radio System (GPRS), or other wireless communication system, such as the Wireless Local Area Network (WLAN), took place all over the world. Various organizations, for example the 3rd Generation Partnership Project (3GPP), the International Telecommunication Union (ITU), 3rd Generation Partnership Project 2 (3GPP2), Internet Engineering Task Force (IETF), and the like, are working on standards for telecommunication network and multiple access environments.
In general, the system structure of such a network is on a client/server basis. In case of a communication network, a subscriber's user equipment, such as a mobile station, a mobile phone, a fixed phone, a personal computer (PC), a laptop, a personal digital assistant (PDA) or the like, is connected via respective transceivers and interfaces, such as an air interface, a wired interface or the like, to an access network subsystem. The access network subsystem controls the communication connection to and from the user equipment and is connected via an interface to a corresponding core or backbone network subsystem. The core (or backbone) network subsystem switches the data transmitted via the communication connection to a destination, such as another user equipment, a service provider (server/proxy), or another communication network. It is to be noted that the core network subsystem may be connected to a plurality of access network subsystems. Depending on the used communication network, the actual network structure may vary, as known for those skilled in the art and defined in respective specifications, for example, for UMTS, GSM, WLAN and the like. Generally, for properly establishing and handling a connection between network elements such as the user equipment and another user terminal, a database, a server, etc., one or more intermediate network elements such as support nodes or service nodes are involved.
TCP/IP (Transmission Control Protocol/Internet Protocol) is a network protocol used on, for example, LANs, WANs and the Internet. With the increasing amount of packet services on wireless systems, the use of TCP on a wireless system increases and in the near future it is believed that a considerable amount of the traffic on a wireless system is based on TCP. Currently a major part of all Internet traffic is based on TCP/IP. In TCP, data is split into what the protocol considers the optimum size chunks to transmit. The chunks are denominated segments and their size must not exceed a maximum constant value (Maximum Segment Size or MSS).
TCP is an end-to-end protocol. Generally, TCP has no advance knowledge of the network characteristics. Thus, it is adapted to adjust the transmission behavior according to the present network conditions. One important feature is the so-called congestion control. Congestion can occur, for example, when data arrives at a network node on a big pipe (fast link) and is to be sent out on a smaller pipe (slower link). Congestion may also occur when multiple input streams arrive at a router or the like whose output capacity is smaller than the total amount of the inputs. In such a case, the incoming data are buffered, for example, in a buffer means of the router or of another network element.
Whenever a segment is transmitted by a server it has to reach a client which acknowledges the transmitted segment. The elapsed time when the server transmits the segment until it receives the corresponding acknowledgment is commonly known as Round Trip Time (RTT). This is shown in FIG. 6 for a segment or packet 1 transmitted from the server to the client and an acknowledgment ACK 1 transmitted from the client to the server. The TCP server holds a variable used to calculate an estimation of a maximum allowed RTT. This variable is called Retransmission Time-Out (RTO). Moreover, the server has a timer that counts the elapsed time since the segment was transmitted. If the corresponding acknowledgment does not arrive at the server before the timer reaches the value of the RTO estimator, the server considers that congestion has occurred in the network and starts congestion avoidance actions.
TCP tries to adapt the transmission rate to the load and capacity of the links of the network. This is done by several mechanisms, like slow start, retransmission timeout, fast retransmission, etc. (see, for example, IETF RFC 2001, “TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms”, by W. Stevens, January 1997). Fast retransmission and retransmission timeout cause retransmission of TCP/IP packets, when they are lost or delayed more than a dynamic timer RTO. Slow start starts the data transmission slowly and increases the window size gradually in order to adjust the size of the sending window to match the current ability of the network to deliver segments (or packets). Slow start is generally used at the start of a connection, at the expiry of the retransmission timer, and/or after a long passive TCP phase.
In the following, TCP performance in cellular networks is contemplated. As mentioned above, TCP is a transport layer protocol that uses rate of arrival of ACKs to gradually utilize network capacity. In addition, TCP starts with a very low throughput during its slow-start period, which occurs at the start of the connections and sometimes, during the connection. In cellular networks, due to low bandwidth and high bit error rates on the radio interface, the rate at which ACKs are received is extremely slow. This means that a TCP sender will require a long time to reach a reasonable throughput during the slow-start period.
One typical implantation for networks using TCP is to use a so-called first-in first-out (FIFO) algorithm for transmitting TCP data and ACK packets in the network. In cellular networks, due to the low link data rate, the network queues may become large. In such cases, the network behavior caused by the FIFO processing may excessively delay the transmission of the ACK so that a TCP sender is prevented from reaching a sufficient throughput in a reasonable time.
As described above, TCP slow start may occur at the start of the connection and whenever there is a timer timeout. The goal of slow start is to start with a small data rate to make sure that the network can accommodate at least a very little amount of data. This is done in order to avoid network congestion. TCP then relies on the rate of arrival of ACKs to gradually increase its data rate until it reaches a reasonable throughput.
In cellular networks, however, due to small bandwidth and high bit error rate, often the round trip time (RTT) of the connection is large. This means that the rate at which ACKs are received is very slow since after sending a window of data the sending side needs to wait one RTT until it receives any acknowledgement. Due to this, the TCP sender may waste a long time in slow-start mode before it can reach a reasonable throughput. This may be experienced by a user as degraded service experience. It is to be noted that in the network all packets of a connection received by a network node which has to forward the data arrived via the connection are queued in (or enqueued) with all packets/data received from other connections arriving at this network node. This means that the RTT during slow start has a strong component of the queuing delay coming from other connections.
There have been made several attempts for solving the problem of accelerating the data transmission rate of connections. In general, either the intention has been to increase the data rate at the start of the connection or to reduce the RTT of a connection.
With regard to the slow start condition of a connection, in order to reduce the time taken by the slow start on long delay paths, it is tried, for example, to change the window increase algorithm of TCP. One example for this is to use a larger window than one packet at the beginning of slow start, wherein an initial window of maximum 4 packets has been proposed. Another example is defined as packet spacing, e.g. Rate Based Spacing, where the source transmits directly at a large window without overloading the network. A further approach on the network side is to establish many parallel TCP connections for the same transfer, which however is problematic when only a limited transfer capacity is available, and which also increases the overall load of the network.
There has been proposed a scheduling/queuing algorithm called Queue Length based Weighted Fair Queuing (QLWFQ) Algorithm. Generally, by means of a weighted fair queuing algorithm, data to be transmitted by one network node and coming from different sources are priorized in order to ensure that each connection is treated in a fair way, i.e. that data of each connection are transmitted in a reasonable time period. In case of the QLWFQ, the packets are scheduled (allocated to a priority) so that the longest queue gets a higher priority. However, the weighted fair queuing is generally independent of connection level information so that changes in a TCP connection are not considered.